(1) Field of the Invention
The present invention relates to an improvement of a non-aqueous electrolyte secondary cell designed for the purpose of improving preservation characteristics and cycle characteristics.
(2) Description of the Prior Art
There has been a rapid reduction in size and weight of mobile information terminals such as mobile phones and notebook personal. As the driving power sources for the terminals, non-aqueous electrolyte secondary cells, which have a high energy density and capacity, are widely used.
In recent years, higher capacity is required of the cells. In view of this, such a positive electrode active material is used that is charged to a higher potential than usual in an attempt to increase the use efficiency of the positive electrode active material.
However, when lithium cobalt oxide, generally used as the positive electrode active material, is charged to a potential higher than 4.3V with the respect to lithium, in particular, cobalt is eluted to the non-aqueous electrolyte in a high-temperature condition, and the eluted cobalt is deposited on a surface of the negative electrode, so that the deposited cobalt exerts a negative effect on the preservation characteristics and cycle characteristics. Also, in a higher potential condition (higher than 4.3V with the respect to lithium), the non-aqueous electrolyte is decomposed easily than in a conventional potential condition. The decomposition of the non-aqueous electrolyte causes generation of gas and then the generated gas hinders a smooth discharge and charge reaction, causing further deterioration in preservation characteristics and cycle characteristics.
In this connection, the techniques intended to contain lithium phosphate in the non-aqueous electrolyte cell for the purpose of improving the cell characteristics are proposed in Patent Documents 1-4 listed below:
Patent Document 1: JP unexamined patent publication No. 2003-308842,
Patent Document 2: JP unexamined patent publication No. 2005-71641,
Patent Document 3: JP unexamined patent publication No. 10-154532, and
Patent Document 4: JP unexamined patent publication No. 9-306547.
However, these techniques have a problem that when the positive electrode is charged to a potential higher than 4.3V with the respect to lithium, the elution of the cobalt and the decomposition of the electrolytic solution cannot be prevented sufficiently.